1. Field of the Invention
The present invention relates to the field of semiconductor integrated circuit (IC) manufacturing, and more specifically, to a method of forming a more reliable under bump metallurgy (UBM) and an UBM that is more reliable.
2. Discussion of Related Art
Chip-to-package interconnections have traditionally involved wirebonding which is very cost-effective. Wirebonding is the use of very fine metal wires to join contacts on a chip with corresponding contacts on a package. As the size of a transistor is reduced, the size of the chip-to-package interconnection also has to be scaled down. However, the performance and the reliability of the chip-to-package interconnection may be affected since wirebonding requires the routing of all the input/output (I/O) connections to the edges of the chip.
Solder bumping is the use of reflowable solder balls to join contacts on the chip with corresponding contacts on the package. Solder bumping requires that the chip be flipped over to face the package. Solder bumping permits I/O connections to be placed across the surface of the chip, which results in many advantages. First, bumping significantly increases the density of the I/O connections. Second, bumping simplifies the design and layout of the chip. Third, bumping decreases the footprint of the package. Fourth, bumping greatly enhances the reliability of the I/O connections.
Transistors on the chip have traditionally been connected with Aluminum lines. As the size of the transistor continued to be reduced, Copper was introduced as a replacement for Aluminum. Copper has lower resistivity than Aluminum so performance of the chip is improved. Copper is more resistant to electromigration than Aluminum so reliability of the chip is also improved.
The bump attached to the bond pad of the chip has traditionally been formed with a Lead-Tin Solder. However, during thermal cycling, the Tin in the bump tends to migrate through cracks or other defects in the UBM and react with the Copper in the bond pad to form an intermetallic compound which may cause shorting of the interconnects thereby leading to premature failure of the interconnect.
Thus, what is needed is a method of forming a more reliable under bump metallurgy (UBM) and an UBM that is more reliable.